The present invention relates generally to precast concrete floor systems and, more specifically, to a precast concrete floor system that has a shallow flat soffit and uses no corbels to reduce the floor height while maximizing useable space.
Conventional hollow-core floor systems consist of hollow-core planks supported by inverted-tee (IT) precast prestressed concrete beams, which are, in turn, supported on column corbels or wall ledges. These floor systems provide a rapidly constructed solution to multi-story buildings that is economical, fire-resistant, and with excellent deflection and vibration characteristics. The top surface of hollow-core floor systems can be a thin non-structural cementitious topping or at least 2 inch thick concrete composite topping that provides a leveled and continuous surface. Despite the advantages of conventional precast hollow-core floor systems, they have the two main limitations of a low span-to-depth ratio and the presence of floor projections, such as column corbels and beam ledges. For a 30 ft bay size, conventional precast hollow-core floor system would require a 28 inch deep IT plus a 2 inch topping, for a total floor depth of 30 inches, which results in a span-to-depth ratio of 12 (PCI, 2010). In addition, this floor would have a 12 inch deep ledge below the hollow-core soffit and a 16 inch deep column corbel below the beam soffit.
On the other hand, post-tensioned cast-in-place concrete slab floor systems can be built with a span-to-depth ratio of 45 and flat soffit, which results in a structural depth of 8 inches for the 30 ft bay size (PTI, 2006). If the structural depth of precast floor systems can come close to that of post-tensioned cast-in-place concrete slab system, then precast concrete systems could be very favorable due to their rapid construction and high product quality. Reducing the depth of structural floor results in reduced floor height, which in turn makes savings in architectural, mechanical and electrical (AME) systems and may allows for additional floors for the same building height. The cost of AME systems is about 75 to 80% of the total initial and operation cost, and any small savings in these systems would have a significant impact on the building life cycle cost.
Low, et al. (1991 and 1996) developed a shallow floor system for multi-story office buildings. The system consists of hollow-core planks, 8 ft wide and 16 inch deep prestressed beams, and single-story precast columns fabricated with full concrete cavities at the floor level. The column reinforcement in this patented system is mechanically spliced at the job site to achieve the continuity (Tadros and Low, 1996). The beam weight and the complexity of the system design and detailing were discouraging to producers.
Thompson and Pessiki, (2004) developed a floor system of inverted tees and double tees with openings in their stems to pass utility ducts. This floor system is appropriate and economical for parking structures as it does not provide either shallow floor or flat soffit required for residential and office buildings.
Hanlon, et al. (2009) developed a total precast floor system for the construction of the nine-story flat-slab building. This system consists of precast concrete stair/elevator cores, prestressed concrete beam-slab units, prestressed concrete rib-slab floor elements; variable-width beam slab; and integrated precast concrete columns with column capital. The need for special forms to fabricate these components and the need for high capacity crane for erection are the main limitations of this system.
Composite Dycore Office Structures (1992) developed the Dycore floor system that consists of shallow soffit beam, Dycore floor slabs, and continuous cast-in-place/precast columns with block outs at the beam level. In this system, precast beams and floor slabs act primarily as stay-in-place forms for major cast-in-place operations required to complete the floor system, which is costly and time consuming.
Simanjuntak, J. H. (1998) developed a shallow ribbed slab configuration without corbels. This is accomplished by threading high tensile steel wire rope through pipes imbedded in the floor system and holes in the columns. The main drawback of that system is the need for false ceiling to cover the unattractive slab ribs.
Wise, H., H. (1973) introduced a method for building reinforced concrete floors, and roofs employing composite concrete flexural construction with little formwork. The bottom layer of the composite concrete floor is formed by using thin prefabricated concrete panels laid side by side in place with their ends resting on temporary or permanent supports. The panels are precast with one or more lattice-type girders or trusses extending lengthwise from each panel having their bottom chords firmly embedded in the panel and with the webbing and top chords extending above the top surface of the panel. The main drawback of that system is the need for shoring during construction, in addition to the limitations of the panel dimensions.
Filigree Widesslap System was presently used under the name of OMNIDEC (Mid-State Filigree Systems, Inc. 1992). It consists of reinforced precast floor panels that serve as permanent formwork. The panels are composite with cast-in-place concrete and contain the reinforcement required in the bottom portion of the slab. They also contain a steel lattice truss, which projects from the top of the precast unit. One of the main advantages for this system is a flat soffit floor which does not required a false ceiling. However, this system requires extensive techniques to produce (Pessiki, et al. 1995).
Bellmunt and Pons (2010) developed a new flooring system which consists of a structural grid of concrete beams with expanded polystyrene (EPS) foams in between. The grid has beams in two directions every 32 inches. The floor is finished with a light paving system on top and a light ceiling system underneath. This system has many advantages, such as lightweight, flat soffit, and thermal insulation. However, some of its disadvantages include the floor thickness, unique fabrication process of EPS forms due to the special connections required.
The Deltabeam (Peikko Group, Peikko News (2010)), is a hollow steel-concrete composite beam made from welded steel plates with holes in the sides. It is completely filled with concrete after installation in site. Deltabeam acts as a composite beam with hollow-core, thin shell slabs, and in-situ casting. Deltabeam can have a fire class rating as high as R120 without additional fire protection. The Deltabeam height varies based on the required span. For a 32 ft span, the Deltabeam can be as shallow as 23 inch (21 inch deep beam+2 inch topping). Although this is 5 inches less than the precast/prestressed concrete inverted tee, it requires shoring for erection, adding shims to the base plate to rise up hollow core to match the level of the top plate, and additional fire protection operations if higher ratings are required.
Although the use of column corbels and beam ledges is the common practice in parking structures and commercial buildings, it is not aesthetically favourable in residential buildings, such as hotels. False ceiling is used in these applications to hide the unattractive floor projections, which results in reduced vertical clearance. Elimination of floor projections combined with shallow structural depth will improve the building aesthetics and overall economics.